WOODAVARD : INAUGURAL ADDRESS. 259 



ganglions are comparatively widely separated, and the connectives and 

 commissures that unite them arc long. Passing to higher and higher 

 representatives, the nerve-centres tend to become more and more 

 concentrated, at first the sensory and motor nerve-centres and then 

 all the others, till they form a ring round the anterior part of the 

 oesopliagus, and finally are intimately united and localized on the 

 doi'sal surface of the latter, as in Pleuroh-anchus, or the ventral side, 

 as in the thecosomatous Pteropods. This progressive advance is 

 observable also in the Cephalopoda, and to a lesser degree in the 

 Pelecypoda and even the Amphineura. 



In the Cephalopoda we meet for the first time in the Mollusca with 

 internal structures of great import, namely, cartiUiges, which are 

 especially developed in the head. In Nautilus there is the H -shaped 

 capito-pedal cartilage, which supports the ventral portion of the nerve- 

 centres, two of its branches extending to tlie base of the funnel. In 

 the Dibranchia the cephalic cartilage completely invests the central 

 nervous system, the oesophagus passing through it. Different Cephalo- 

 poda have additional cartilaginous pieces in other parts of the body, 

 such as the bases of the fins, and the arms, at the base of the neck 

 (when the mantle is not fused to the head), at tlie internal extremities 

 of the retractor muscles of tlie head and funnel, and even in the two 

 branchial lamellae. Here we have the bases of a possible internal 

 skeleton that might conceivably be called into existence by the 

 operation of circumstances at present unforeseen, but seeing that, 

 despite the long geological ages of their existence, the Cephalopoda 

 are still behind the earliest known fishes in this respect, much time 

 must elapse before its evolution, and the further chronicle of any 

 progress in this respect will fall to others than ourselves. 

 Summary. 



Despite the plastic nature of the Mollusca, progressive development 

 is traceable in certain characters, while in other conspicuous features 

 the action of environment, or individual requirement, alone seems 

 responsible. 



In all the Classes there is a tendency to get rid of the shell, 

 apparently as the result of the assumption of more active habits, 

 especially among the carnivorous individuals. 



The form of the body and the adaptations of the foot would appear 

 to be solely influenced by considerations of habit and habitat. 



In the Radula, on the other hand, there is consistent progress in 

 the shape of the replacement of numerous, weak, little teeth by few, 

 strong ones, especially in the carnivorous groups. 



The Circulatory System shows advance from a diffuse form in the 

 archaic to a well-defined one in the highest tribes. 



The Respiratory System, per contra, develops in response to 

 individual requirements rather than on any well-determined lines of 

 progress. 



Finally, the Nervous System shows distinct advance from the 

 dispersed character that obtains in the primitive groups up to the 

 concentrated form that it assumes in the highest types. 



